The girl's mothers half-brother, because if you think of it mother's brother is like her uncle. Amar's mother's father is his Grandfather. His grand father only had one child which was her uncle but her mom doesn't have the same father as the uncle but has two dad's and one mother.

They are brother and sister...her mother's brother (uncle) is the only son of Amar's grandfather (uncle). So they are either sibling (same mother) or they would be cousins (different mothers, only bc they didnt specify how many daughters/children)